1. Field of the Invention
The present invention is directed to a magnetic resonance apparatus of the type having a gradient coil system that includes a gradient coil arrangement with a conductor and magnetostrictive material, and having a basic field magnet system for generating a basic magnetic field.
2. Description of the Prior Art
Magnetic resonance tomography is a known technique for producing images of the inside of the body of an examination subject. To this end, rapidly switched gradient fields that are generated by a gradient coil system are superimposed on a static basic magnetic field in the magnetic resonance apparatus. The static field is generated by a basic field magnet system. The magnetic resonance apparatus also comprises a radio-frequency system that emits radio-frequency signals into the examination subject for producing magnetic resonance signals and that picks up the generated magnetic resonance signals, from which magnetic resonance images are produced.
For generating gradient fields, suitable currents are to be set in the gradient coils. The amplitudes of the required currents amount to several 100 A. The current rise and decay rates amount to several 100 kA/s. Given the presence of a basic magnetic field on the order of magnitude of 1 T, Lorentz forces that lead to oscillations of the gradient coil system act on these temporally variable currents in the gradient coils. These oscillations are forwarded to the surface of the apparatus via various propagation paths. At this surface, these mechanical oscillations are converted into acoustic oscillations that ultimately lead to noise that is disturbing.
For reducing such noise, German OS 196 43 116, corresponding to U.S. Pat. No. 5,952,829, discloses that forces that oppose the Lorentz forces be generated by magnetostriction. Such opposing forces are thereby generated by the same gradient field that also produces the Lorentz forces. This published application proposes for this purpose given a hollow-cylindrical gradient coil system, to provide a magnetostrictive material system composed of planar or strip-shaped, magnetostrictive components, with the gradient coil system, preferably the in longitudinal direction, parallel to the principal axis of the hollow cylinder and/or in circumferential direction. It is primarily the bending vibrations of the hollow cylinder produced by transverse gradient coils that are responsible for the noise. For suppressing these vibrations, it is proposed in this published application that four strip-shaped, magnetostrictive longitudinal elements, that are equally spaced in the circumferential direction, be secured to the hollow-cylindrical gradient coil system. It is especially disadvantageous in this solution that the four strip-shaped longitudinal elements represent large eddy current surfaces. Moreover, a significant amount of magnetostrictive material must be utilized overall for an adequate noise reduction.
It is an object of the present invention to provide a magnetic resonance apparatus of the type initially described that alleviates the aforementioned disadvantages of known systems of this type.
This object is inventively achieved in a magnetic resonance apparatus wherein magnetostrictive material is arranged in immediate proximity to the conductor in areas having the same course as the course of the conductor of the gradient coil system. As a result, the magnetostrictive material is arranged at those locations at which it experiences a large change in length due to its spatial proximity to the conductor, and thus to a conductor-proximate magnetic field that is produced in the immediate proximity of the conductor by a current flowing in the conductor. The noise-reducing effect is therefore considerable. At the same time, a large-area arrangement of magnetostrictive material is avoided, so that only a small amount of eddy currents can still form in the magnetostrictive material.
In an embodiment, the magnetostrictive material is arranged in areas wherein a section of the conductor has a longitudinal direction substantially perpendicular to the basic magnetic field. A xe2x80x9csectionxe2x80x9d means an imaginary subdivision of the conductor in the longitudinal direction of the conductor, and the longitudinal direction of the section means the direction that a current in the conductor has in the middle of the section.
As a result, the magnetostrictive material is arranged exclusively at those locations at which Lorentz forces act on the conductor, a high noise-reducing effect being thus achieved given a minimal use of material.
In an embodiment, the magnetostrictive material is arranged in areas wherein a conductor-proximate magnetic field, that is produced in the immediate proximity of the conductor by the current therein, has a field component that is co-linear with the basic magnetic field. As a result, a designational length change of the magnetostrictive material is achieved in the direction of the basic magnetic field, so that, for example, a pre-polarization or pre-stressing of the material by the basic magnetic field can be employed.
In another embodiment, the magnetostrictive material is arranged in regions opposite one another with respect to a cross-section of the conductor, such that the material acts with a positive magnetostrictive effect in one of the regions and acts with a negative (opposite) magnetostrictive effect in the region lying opposite. A positive magnetostrictive effect means that the material exhibits an increase in length in the direction of the magnetic field given an increase in a magnetic field permeating the material. Material experiencing a negative magnetostrictive effect exhibits a decrease in length given such an increase in the permeating magnetic field. The noise-reducing effect is intensified by the use of materials with positive and negative magnetostrictive behavior, respectively.
In a further embodiment for a transverse gradient coil arrangement of a hollow-cylindrical gradient coil system with at least one sub-coil fashioned saddle-shaped, the magnetostrictive material is arranged such that the quantity of the material is maximum in the region of a saddleback or ridge line of the sub-coil, and decreases toward the edges of the sub-coil proceeding from the saddleback line. As a result, the banana-shaped bending vibration that is mainly responsible for noise in such a hollow-cylindrical gradient coil system is suppressed and, at the same time, an undesired oscillation in the circumferential direction of the hollow cylinder is prevented from occurring.
In another embodiment, the magnetostrictive material is arranged finely distributed in an electrically non-conductive matrix. As a result, formation of eddy currents in the magnetostrictive material is nearly completely suppressed, and thus disturbing influences on results of a magnetic resonance examination are avoided.
In another embodiment, the magnetostrictive material is fashioned such that a pre-polarization of the material by the basic magnetic field effects an operating range of the material in the predominantly linear region of a flux density length change characteristic of the material. As a result, a uniformly high, noise reducing effect of the magnetostrictive material is achieved independently of the size and direction of the current changing over time in the gradient coil arrangement.